1. Field of the invention
This invention relates to improvements in thermochemical treatment of steel components designed to produce on the surface of the components a layer capable of withstanding corrosion attack for an extended period of time.
2. Brief description of the prior art
In the prior art, various oxidizing treatments are known and commonly used to produce on the surface of previously nitrided or nitro-carburized components, a thin layer of oxides predominantly made-up of Fe.sub.3 O.sub.4, usually less than 1 micron in thickness. This objective is obtained either by immersing the previously hardened (nitrided) components in toxic oxidizing salts or by exposing these components to a controlled oxidizing atmosphere. These known methods are efficient but have serious drawbacks. Indeed, when the hardening and oxidizing treatment is carried out in salts, it usually involves first hardening in potassium cyanide/cyanate bath, followed by water quenching and subsequent polishing and reoxidizing in a separate bath. Salt bath treatment poses serious environmental and health problems and involves multiple processing stages, rather awkward in serial production. Moreover, it does not offer an adequate corrosion protection.
In other development as described in U.S. Pat. No. 4,496,401, the steel components are hardened by a ferritic nitrocarburizing process and subsequently subjected to an oxidizing atmosphere for a limited period of time. The oxidation takes place usually in the air and is followed by a rapid quench This treatment allows the formation of a nitrogen diffusion zone followed by a layer of .epsilon. iron nitride or carbonitride and by another oxide-rich superficial layer impregnated of oil or wax, on the surfaces of the steel components Other variation of this process involves polishing and reoxidizing at different temperature followed possibly by a quench.
It is felt that processing of components in such a manner has also some major disadvantages, namely high processing temperatures, thick and relatively brittle superficial layer as well as uncontrolled oxidizing conditions in the free air.
U.S. Pat. No. 4,391,654 describes a process especially designed for high speed cutting tools, which basically consists in subjecting the steel component to a preliminary oxidation before subjecting it to hardening, which allows the formation of a nitrogen diffusion zone onto the surface of the steel component while eliminating the simultaneous formation of superficial .epsilon. or .gamma.' iron nitride or carbonitride layers.